In the past, various attempts have been made to treat and/or recycle spent potlining from aluminum reduction cells. For instance, U.S. Pat. No. 5,286,274 to Lindkvist, et al., which is incorporated herein by reference in its entirety, is directed to a method for the treatment of potlining residue from primary aluminum smelters. Lindkvist, et al. is directed to supplying a closed electrothermic smelting furnace with crushed spent pot liner and an SiO2 source. The spent pot liner residue after oxidation is melted at temperatures from 1,300xc2x0 C. to 1,750xc2x0 C. Other additions to the smelting furnace include an oxidation agent to oxidize carbon and other oxidizable components such as carbide along with a source of calcium oxide to react with all the flourine present to form CaF2, calcium almuniate or calcium aluminate silicate slag.
Various deficiencies and drawbacks, however, remain in the prior art regarding the recycling of spent pot liner. In particular, many processes are simply not well suited to removing all of the impurities contained within the waste materials in order to produce a viable product. Also, many prior art processes produce waste by-products that must be subsequently treated. The present invention addresses these and other disadvantages of the prior art and provides various improvements as will be made apparent from the following description.
The present invention recognizes and addresses the foregoing disadvantages and others of the prior art. Accordingly, it is an object of the present invention to provide an improved process for recycling spent pot liner, especially pot liner obtained from aluminum electrolytic cells.
Another object of the present invention is to provide a process for recycling spent pot liner by converting the carbon contained in the pot liner to silicon carbide.
Still another object of the present invention is to provide a process for recycling spent pot liner that not only converts carbon into silicon carbide, but also decomposes flourine containing compounds and cyanates.
These and other objects of the present invention are achieved by providing a process for recycling spent pot liner obtained from aluminum electrolytic cells. The spent pot liner can contain various materials, depending upon the conditions under which the pot liner was used. For most applications, the pot liner will contain carbon, cryolite (Na3AlF6), cyanide compounds including sodium cyanate and/or potassium cyanate, silica, various metals including aluminum, iron and magnesium, aluminum oxide, besides various other materials.
According to the process of the present invention, the spent pot liner is first comminuted using, for instance, a crushing device. For most applications, the spent pot liner should be comminuted to an extent such that at least 90 percent of the particles by volume have a diameter of less than about 1 millimeter.
After being comminuted, the spent pot liner is then combined with silica. The silica can be added in an amount such that the molar ratio between the total silica present in the pot liner and carbon is from about 1:3 to about 1:5, and particularly from about 1:3 to about 1:4. The silica that is combined with the pot liner can be obtained from various sources. For instance, silica can be added to the spent pot liner as sand, fumed silica, fly ash, clay, and/or metal silicones.
The spent pot liner and silica are then heated under conditions that promote the formation of silicon carbide. For example, preferably the carbon and silica are heated by an electrical resistance furnace in a reducing atmosphere and in an oxygen starved atmosphere. As used herein, an oxygen starved atmosphere refers to an atmosphere containing less than 5 percent molecular oxygen. For most applications, the spent pot liner and silica are heated to a temperature of at least 1800xc2x0 C., and particularly from about 1800xc2x0 C. to about 2200xc2x0 C.
During formation of silicon carbide, flourine containing compounds and cyanides will decompose and exit the furnace in the flue gas stream. It is believed that for most applications, flourine will volatilize and reform in the flue gas stream as hydroflouric acid which can then be recovered from the flue gas stream using an appropriate wet or dry scrubber.
After heating, the formed silicon carbide can be separated from the remainder of any other materials and reused as desired. In order to separate the silicon carbide from the other materials, various processes can be used, such as gravimetric separation techniques.
Other objects, features and aspects of the present invention are discussed in greater detail below.